I. Field of the Invention
This invention relates generally to an implantable cardiac rhythm management apparatus, and more particularly to an improved device and method of operating the device to make it efficacious in the treatment of congestive heart failure.
II. Discussion of the Prior Art
Congestive heart failure (CHF) is an insidious disease affecting at least two million Americans. Patients diagnosed with heart failure have such a poor long-term prognosis (the average life span is five years) that it is tantamount to a death sentence. Thus, the potential market for successful therapy is not only large but highly motivated.
In the past, implantable cardiac pacemakers have principally been used to treat heart rate disorders, including bradycardia and tachycardia. Marguerite Hochleitner and her co-workers in the Department of Medicine of the University of Innsbruck published a paper called "Usefulness of Physiologic Dual-Chamber Pacing in Drug-Resistant Idiopathic Dilated Cardiomyopathy" in the American Journal of Cardiology (Vol. 66, Jul. 15, 1990, pp. 198-202) describing a study on a series of patients as to the beneficial effects of physiologic, dual-chamber (DDD) pacing in the treatment of end-stage idiopathic dilated cardiomyopathy. The pacemakers were implanted with the atrial electrode positioned near the right auricle and the ventricular electrode in the apical region of the right ventricle. Externally microprogrammable pacemakers, allowing reprogramming of the atrioventricular (AV) interval, were used. The AV interval was set at 100 milliseconds, this value being chosen as the shortest possible AV delay which would not significantly impair cardiac function. A marked improvement in cardiac function in most of the patients in the study resulted. For example, a remarkable improvement was noted in left ventricular ejection fraction, a reduction in heart size, a decrease in heart rate with tachycardia patients and an increase in heart rate for patients with bradycardia.
The devices described in the Hochleitner paper are not capable of adjusting the AV interval to fit the patient's changing condition after implant. While they do allow a change in AV interval through external reprogramming by the physician, optimization of AV interval through closed loop control is not involved. Moreover, those devices did not include a means for converting a fibrillating heart to sinus rhythm.
Current dual chamber pacemakers that allow for self-adjustment of the AV interval use an algorithm based entirely on rate. This shortening of AV interval with increased rate is merely designed to mimic the shortening of the PR interval which occurs in normal individuals with increases in rate. Patients in heart failure, however, will require adjustments in AV interval which are independent of rate and are, instead, a function of the patient's condition, posture, etc. Moreover, the AV shortening required with rate increases in these patients may not be identical to that for normal individuals.
In a paper entitled "Hemodynamic Effect of Physiological Dual Chamber Pacing in a Patient with End-Stage Dilated Cardiopathy: A Case Report" by Kataoka (Pace, Vol. 14, September 1991, pp. 1330-1335) describes the treatment of a patient in CHF using a DDD pacemaker. Again, the pacemaker described in this paper, while adjustable via an external transcutaneous programmer, did not provide for self-adjustment to meet changing conditions of the patient. Kataoka recommended that invasive hemodynamic measurements and a Doppler flow study be carried out to define an optimal AV interval but, as mentioned, the system described therein is basically open-loop with no feedback being provided for automatically adjusting AV interval to a value that optimizes systolic and diastolic function. None of the reported devices incorporate arrythmia detection and/or defibrillation backup.